1. Field of the Invention
This invention relates to in situ doping of polycrystalline silicon with phosphorous trichloride, tertiary butyl phosphine, isobutyl phosphine, trimethyl phosphate and tetramethyl phosphate.
2. Brief Description of the Prior Art
In situ doping of polycrystalline silicon (polysilicon) is known in the art, such doping generally using phosphine as the phosphorous containing dopant and diborane as the boron containing dopant. However, special techniques are required when phosphine is the dopant to improve sheet resistance and thickness uniformity and to achieve an acceptable deposition rate. Also, the prior art phosphorous-containing dopants are highly toxic and therefore require a great deal of care in handling. This prior art is set forth in an article by B. S. Meyerson and M. L. Yu, ECS Extended Abstracts 83-1. 651(1983), an article by D. L. Flowers in the same publication 84-1. 256(1984) and an article of H. Kurokawa, Journal of the Electrochemical Society, 129.26.20(1982).
In situ doping of polysilicon is attractive because the dopant distribution is uniform throughout the film if the dopant is incorporated during and along with the deposition of the polysilicon. This dopant uniformity is extremely advantageous in the refilling of trenches with polysilicon since doping concentrations will then be the same throughout the entire trench, thereby eliminating the need for complicated implants and anneals currently used to fill trenches.
In situ doping differs from other prior art doping methods, such as ion implantation and solid solid source diffusion in a furnace tube, wherein the dopant is put down interstitially with the polysilicon and simultaneously therewith. This provides the uniform build-up of the polysilicon film with the dopant uniformly distributed therein. However, as stated above, the dopants utilized in the prior art have been extremely toxic and have led to the requirement that measures be taken to protect against the fumes and other effluent thereof, this requiring a great deal of expense. It is therefore a need of the art to provide a dopant for use in the in situ doping process which does not have the toxic effect of prior art dopants, yet can provide the same results.